Methyl-palladium(II) complexes of pyridine-bridged bis(nucleophilic heterocyclic carbene) ligands: Substituent effects on structure, stability, and catalytic performance

Abstract

A series of discrete, mononuclear palladium(II)–methyl complexes, together with several palladium(II)–chloro analogues, of pyridine-functionalised bis-NHC ligands have been prepared via ligand transmetallation from the silver(I)-NHC complexes. The reported complexes comprise examples with both the methylene-bridged 2,6-bis[(3-R-imidazolin-2-yliden-1-yl)methyl]pyridine ( RC ∧N ∧C; R = Mes, dipp, t Bu) and planar 2,6-bis(3-R-imidazolin-2-yliden-1-yl)pyridine ( RCNC; R = Mes, dipp) ligands and, when combined with the previously reported MeC ∧N ∧C/ MeCNC examples, cover a broad spectrum of ligand substituent steric and electronic properties, including the bulky Mes and dipp groups frequently used in catalytic applications. The palladium(II) complexes have been characterised by a variety of methods, including single crystal X-ray crystallography, with the shielding of the Pd– Me groups in the proton NMR spectra of some of the N-aryl substituted examples correlated with the proximity of the aryl rings to the methyl group in the solid state structures. The [PdMe( RC ∧N ∧C/ RCNC)] + complexes undergo thermal degradation via reductive methyl-NHC coupling to give 2-methyl-3-R-imidazolium-1-yl species with relative stabilities in the order of [PdMe( MesC ∧N ∧C)]BF 4 > [PdMe( MeC ∧N ∧C)]BF 4 ≈ [PdMe( MesCNC)]BF 4 > [PdMe( MeCNC)]BF 4 > [PdMe( tBu C ∧N ∧C)]BF 4 ≫ [PdMe( tBu CNC)]BF 4 (not isolable). A comparison of the activity of the complexes as precatalysts in a model Heck coupling reaction shows greatest activity in those species bearing bulkier N-substituents, with complexes bearing RC ∧N ∧C ligands generally more efficient precatalysts than those bearing RCNC ligands.